Fix --payload for optional stateful.tgz.
Fix --payload for optional stateful.tgz.

Change-Id: I0af77b0ef83b016150aecdf6947182df8bad042d

BUG=chromium-os:9492
TEST=Ran manually

Review URL: http://codereview.chromium.org/5281002

coreos/dev-util,marineam/coreos-dev-util,coreos/dev-util,mgireesh05/dev-util,mgireesh05/dev-util,marineam/coreos-dev-util,mgireesh05/dev-util,coreos/dev-util